Numerous disclosures have been made in the prior art with the aim of improving the initial discharge voltage of the lithium/fluorinated carbon batteries. Illustrative of such prior art efforts to eliminate or reduce initial voltage suppression are the disclosures in the following references wherein admixtures of different fluorinated carbonaceous materials are used for the purpose of achieving some improvement in discharge voltage:
Japanese Kokai No. 82 84,570 discloses a mixture of fluorinated carbon (CF.sub.n) and fluorinated graphite (C.sub.2 F).sub.n with the relative proportions of (C.sub.2 F).sub.n ranging from 10:100 to 100:100. Mixtures of (C.sub.2 F).sub.n and fluorinated carbon are disclosed as giving lithium batteries with improved discharge characteristics. The (C.sub.2 F).sub.n additive in that reference is distinguished from fluorinated carbon (CF).sub.n (i.e., CF.sub.x) by its stoichiometry and completely different crystal structure. These differences in structure are described in U.S. Pat. No. 4,139,474. (C.sub.2 F).sub.n must be prepared by reacting graphite for long times with fluorine with 100-200 hours being typical reaction times. The coke based CF.sub.x with which the present invention is concerned, and which is distinct from the fluorinated graphite of Japanese Kokai No. 82 84,570 can be made in less than 24 hours by reacting the coke starting material with fluorine at from 300.degree. C. to 600.degree. C. Use of the coke-based material, CF.sub.x is otherwise relatively generally advantageous over the graphite-based material, (C.sub.2 F).sub.n.
In U.S. Pat. No. 4,139,474, (C.sub.2 F).sub.n is distinguished in several respects from CF.sub.x ; by X-ray diffraction (see line 57, col. 4): IR (see line 59, col. 5); ESCA (see line 31, col. 5), and specific gravity (see line 26, col. 6). Other physical and chemical differences are also enumerated at line 32, column 6, et seq. The starting carbon which must be used to prepare (C.sub.2 F).sub.n is described at col. 8, line 26 et. seq. That patent specifies that any carbon having a Franklin's P-value of from 0 to about 0.6 will be acceptable starting material from which (C.sub.2 F).sub.n can be prepared. The Franklin's P-values for the cokes used in the present invention, see examples I, II and IV are primarily outside this range. Furthermore, as seen at column 8, starting at line 43 of U.S. Pat. No. 4,139,474 natural graphite is the preferred starting carbon to prepare (C.sub.2 F).sub.n and that while petroleum cokes can be used, they must be heat-treated (or calcined) at temperatures between 2000.degree. C. and 3000.degree. C. In the present invention, the materials are petroleum-based coke products prepared normally by heat treatment below 2000.degree. C., or at most about a maximum of about 2000.degree. C. (Example III). Basis supporting a distinction of the invention from Japanese Kokai No. 82 84,570 is also found in Kirk-Othmer's Encyclopedia of Chemical Technology, page 557, wherein the differences between carbon and graphite are described. Carbon, which includes coke, is made from carbonaceous material which is heat-treated at temperatures between 800.degree. C. and 1400.degree. C.; graphite is prepared by further heat treatment at temperatures in excess of 2400 degrees. In the present invention, the starting carbons must be carbonaceous materials heat-treated at a temperature between about 800.degree. C. and 2000.degree. C. Further, as indicated by reference to U.S. Pat. No. 4,139,474 particularly concerning the starting carbons used in that patent, it is clear that the carbons used in Japanese Reference No. 82 84,570 are clearly different from the petroleum cokes employed in the invention.
The disclosures in the above-mentioned Japanese Kokai No. (82 84,570) indicate that the addition of amounts (C.sub.2 F).sub.n to (CF).sub.n improves the drop in the initial voltage over that of (CF).sub.n alone, because the two materials are similar in nature and discharge via similar mechanisms. We have discovered, however, that undesirable results in the initial stages of the discharge curve still occur when similar materials are blended and that other and more important criteria must be used to guide the selection of the best additive material to suitably eliminate the undesirable initial voltage suppression. Accordingly, it is important according to our discovery that the CF.sub.x material added to the bulk CF.sub.x material be different from the bulk material.
Japanese Kokai No. 77 10,519 discloses that graphite fluoride which reacts with potassium iodide (KI) to produce greater than 1.25.times.10.sup.-6 mole of free iodine (I) per gram of CF.sub.x can be used in a cell and that this material gives a cell which does not undergo voltage lowering (i.e., voltage suppression) during the initial part of discharge. The example given in that reference is for a graphite fluoride which liberates 4.times.10.sup.-6 mole of I per gram of material. This reference does not disclose the desirability of blending this material with CF.sub.x nor does it address the deleterious effects of using a material with such a large content of a highly oxidizing species in a lithium battery. In fact, a later Japanese Kokai No. 77 21,621 to the same assignee claims that a graphite fluoride which reacts with KI to give less than 1.25.times.10.sup.-6 mole of I per gram of material can be used in a lithium cell to give one with good voltage after storage. In the example described, two cells are compared; one made from a graphite fluoride with liberates 2.9.times.10.sup.-7 mole of I per gram and one that liberates 1.times.10.sup.-6 mole of I per gram of material. After storage at 70.degree. C. for 1 month, the cells were discharged at 20.degree. C. on a 8 ohm load. After 10 hours, the first cell gave 2.3 V compared to the second cell which gave 1.3 V. In light of Japanese Kokai No. 77 21,621, the graphite fluoride described in Japanese Kokai No. 77 10,519 may eliminate voltage suppression but suffers from the fact that a long lasting, i.e., durable lithium battery cannot be made with it.
Japanese Kokai No. 84 31,564 discloses the use of carbon containing adsorbed fluorine in the CF.sub.x electrode to replace the carbons which are typically used to enhance the electronic conductivity of the electrode matrix. Adsorbed fluorine, because of its strongly oxidizing nature, may be very deleterious and is regarded as disadvantageous.
The use of fluorinated carbon black is disclosed in Japanese Kokai No. 83 05,967. The carbon black which is prescribed is that which has a surface area greater than 800 m2/g. Use of this material in mixtures with CF.sub.x is disclosed as improving the initial discharge voltage of the lithium cell. However, N. Watanabe, et al., SOLID STATE IONICS, page 503 (1980), has shown that fluorinated carbon blacks perform more poorly, particularly at high rates of discharge, than do coke-based fluorinated materials. In light of this disclosure, it cannot be expected that fluorinated carbon black would be as effective in curing the voltage suppression phenomenon as is a coke-based fluorinated material.
Japanese Kokai No. 83 206,057 discloses the incorporation of a noncovalent graphite fluoride intercalation compound into a graphite fluoride cathode for the purpose of improving the initial discharge voltage. Examples are graphite intercalates of AlF.sub.3 or MgF.sub.2 and fluorine. The disclosure of this publication differs from the present invention in that the additive is a noncovalent intercalation compound of graphite and that the major cathode active material is graphite fluoride as distinguished from fluorinated carbon.
Other Japanese patent publications disclose the use of a variety of materials which do not contain fluorine and have been claimed to eliminate or reduce the severity of voltage suppression in Li/CF.sub.x cells. Thus, Japanese Kokai No. 83 123,663 discloses the addition of aluminum powder; Japanese Kokai No. 83 206,058 discloses the addition of aluminum fiber to CF.sub.x cathode compositions for the purpose of stabilizing the initial discharge voltage; and Japanese Kokai No. 83 161,260 discloses the addition of compounds of the form Cr.sub.x O.sub.y where 15&lt;y/x&lt;3. Examples given are Cr.sub.3 O.sub.6 and Cr.sub.2 O.sub.5 and these additives are claimed to improve the initial drop in cell voltage. In each case, a metal or metal salt is added to the cell and therefore differ from our invention.
Additionally, Japanese Kokai No. 82 124,865 discloses that Li/CF.sub.x cells can be partially discharged (.ltoreq.10% of the cell capacity is removed) to improve the initial discharge voltage. This practice minimizes the problem but suffers from several disadvantages; an additional step is required in the production process, processing time is increased, and additional equipment is required.
In summary, with respect to the prior art, while it is known that various blends and admixtures of fluorinated carbon compositions have been employed to improve performance, a need exists for an improved practical fluorinated carbon composition which substantially and effectively eliminates the voltage suppression which occurs in the initial discharge of Li/CF.sub.x batteries.